Once upon a time, a deadly plague caused by Corrupted Blood swept through Azeroth, an event that would later be known as the Corrupted Blood Incident. It all started on September 13, 2005... On that day, update 1.7 Rise of the Blood God was released, which featured a new raid called Zul'Gurub. His final boss was Hakkar the Soulflayer and he had the ability Corrupted Blood. The spell was intended to infect Hakkar's enemies and deal damage to them over time, it was also intended to transfer to other nearby characters. However, the curse should last a few seconds and only work in the raid area. However, a bug crept into the game code and the plague escaped from Zul'Gurub...

Hakkar the Soulflayer

The spell was meant for characters with the highest level, so low level players were dropping like flies. The disease was also spread by NPCs, who didn't die from it, but were carriers, so they could pass the disease on. Locations that were very popular suddenly became deserted. Cities teeming with life turned into skeleton-strewn wastelands. Characters dying in World of Warcraft could come back to life quickly, but still, death exposed players to financial losses, virtual financial losses of course.   

Blizzard tried to fix the bug by introducing a so-called hotfix, then tried to persuade players to temporarily quarantine. However, that didn't work, so they decided to completely shut down the servers and make changes to the code that fixed the bug.

The case was so serious that epidemiologists and anti-terrorist services became interested in it. Scientists studied how people reacted to the raging plague. Admittedly, the spell was poorly designed, but there wouldn't be an epidemic if it wasn't for the intentional spreading of the spell in cities by malicious players. The Tainted Blood could only get outside the raid walls by infecting the hunter's pet first. The infected pet was then specifically allowed into, say, a city and infected others there. It was noticed that the most common reaction to the prevailing disease was running away and isolation from other characters. Some players with healing skills tried to help the infected, some did it for financial gain. There was also a group of people who visited infected areas out of curiosity and to see deserted cities.

The Tainted Blood Incident still stands today as the biggest virtual outbreak in history. World of Warcraft at that time was only ten months old, but it already managed to gather four million active players. This unplanned experiment provided a lot of material for research, thanks to which scientists could test the behavior of modern society in the face of a pandemic.